Specifically, it’s an income-inequality and economic-mobility problem. The steadily dropping marriage rate both contributes to income inequality and further entrenches it.

The latest numbers, from the Pew Research Center, are startling and disturbing. In 1960, nearly three-fourths of those 18 and older were married. By 2010, that number had plummeted to a bare majority, 51 percent. Four in 10 births were to unmarried women.

In 1960, the most- and least-educated adults were equally likely to be married. Now, nearly two-thirds of college graduates are married, compared with less than half of those with a high school diploma or less. Those with less education are less likely to ever marry and more likely to divorce if they do.

“Family structure is a new dividing line in American society,” Isabel Sawhill of the Brookings Institution told me.

As marriage increasingly becomes a phenomenon of the better-off and better-educated, the incomes of two-earner married couples diverge more and more from those of struggling single adults. There is a chicken-and-egg conundrum at work here: Did lack of financial stability contribute to the decision not to marry, or did the decision not to marry contribute to financial instability? Either way, the phenomenon is self-reinforcing.

Of even more concern is the generational impact of this increased inequality. Being raised in a stable, two-parent household is a strong determinant of educational achievement. In turn, educational achievement is a strong — and growing stronger — determinant of lifetime income. As a result, the marriage gap becomes a grimly self-perpetuating process.

Rhapsodizing about the benefits of marriage may have a conservative air — promoting marriage among welfare recipients was a big deal during the George W. Bush administration — but you don’t have to be a conservative to bemoan these statistics.

It’s not only that those at higher education levels are far more likely to marry — they’re far more likely to marry each other. “Men used to marry their secretaries,” Sawhill observed. “Now they marry the woman they met in med school.”

As a result, Sawhill said, “These two-earner couples at the top are just making out like bandits and these single parents at the bottom have miserable lives. If the single parents were married, their life wouldn’t be so miserable. And at the top, if these high-earning professionals weren’t getting together and forming little collaboratives, they’d be worse off.”

About those collaboratives: More people are cohabiting these days, but as an economic matter, this doesn’t solve the problem.

An earlier Pew study found that the typical college-educated cohabiter enjoyed a slightly higher household income than a college-educated married person, which only makes sense. For the college-educated cohabiter, living together tends to be a step toward marriage and children, at which point household income may drop as one spouse works less.

But for those who aren’t college-educated, cohabitation is more an alternative to the marriage track than a precursor of it. They are more far more likely than college-educated cohabiters to have children — and they enjoy significantly lower median household incomes than comparably educated married couples.

Not only that, cohabitation is not the equivalent of marriage in terms of family stability. Demographers Sheela Kennedy and Larry Bumpass found that, by age 12, about two-thirds of children born to cohabiting parents will see them split up, compared with a quarter of children born to parents who are married.

Nor does the marriage gap seem destined to lessen. Pew found that 27 percent of those with college degrees say they consider marriage “obsolete.” But 45 percent of those with a high school diploma or less took that view.

A different arm of Pew, its Economic Mobility Project, found that among children who started in the bottom third of income, only one-fourth of those with divorced parents moved up to the middle or top third as adults. By comparison, half of children with continuously married parents — and, somewhat surprisingly, 42 percent of those born to unmarried mothers — moved up the income ladder as adults.

Is marriage a magic-bullet solution to the broader problem of income inequality and lack of economic mobility? No, but fewer marriages will mean more inequality. Neither development is healthy.